Fruit flies, octopuses, birds, and humans don’t seem to have much in common. Some live on land, others are aquatic. Some fly, while others are tied to the ground. Some are vertebrates, others have no spine. These creatures have evolved separately, and their common ancestors are far, far back in the evolutionary chain. But I can share one fundamental trait: dreaming.
Almost all creatures sleep, although there is some debate about the existence of single-celled organisms, such as the paramecium. But no one really knows Why. For years, researchers have been revealing theories that sleep helps with memory, growth and learning – and it is clear that people need sleep to function properly – but there are few other things that are well understood. “Sleep is this big black box,” says Marcos Frank, a neurologist at Washington State University. Frank compares sleep to a mysterious organ: it is clear that it exists and is vital to animal health, but the exact function and mechanisms that control it are still unknown.
It is even more mystifying that some species seem to have a single state of sleep, while their brains are relatively calm, while others seem to experience two kinds, a quiet phase and an active state. In humans, the period in which the brain lights up with activity is called rapid eye movement sleep (REM). It is when we dream and when we are the hardest to wake up.
For a long time, scientists had not observed this phase of deeper active sleep in amphibians or reptiles. So, until recently, the theory was that it evolved later in history, through an ancestor shared by birds and animals. But in 2016, active sleep was recorded in lizards. Then, in 2019, the state was described in sepia, and in March, a team of scientists from Brazil published a paper in iScience identifying him in octopuses. Cephalopods like these eons evolved before the appearance of creatures that would have shared a lineage with both birds and humans. “There’s no way there’s a common ancestor there,” says Frank. Now scientists are wondering if this state of sleep is more common than they originally realized or if it developed in different species at different times, how wings and flight appeared separately in insects, bats and birds, a phenomenon called convergent evolution.
Understanding what selective pressure caused this adaptation and conserving the genes encoding it could help scientists understand what function dreaming serves for the central nervous system and why sleep is important. “What does animal sleep do?” asks Sidarta Ribeiro, co-author of the newspaper and director of the Brain Institute at the Federal University of Rio Grande do Norte.
The first step in studying how animals sleep is to figure out when they are actually asleep. This is more complicated than it seems. “Imagine you were on Mars and found an organism,” says Frank. “How would you know if he fell asleep or not?”
For mammals, scientists could implant electrodes in their brains to track how their neurons shoot. But octopuses have a very distributed central nervous system. Instead of concentrating control of the nervous system in a brain, they have eight lymph nodes in their arms that often act independently.
Instead of using an invasive method, such as attaching probes to determine the sleep states of octopuses, scientists at the Ribeiro Institute studied some of their behavioral characteristics. Sylvia Medeiros, a graduate student and lead author of the study, tested the excitement thresholds of the animals. Three of the four octopuses in the lab received a visual stimulus – a video with moving crabs. One received a vibrating stimulus in the form of a light pressure on the tank. Medeiros wanted to see how quickly they responded to stimuli when they were awake. He then tested them when they appeared inactive and measured their response rates. Slower reactions meant they fell deeper asleep.